Vehicle arresting bed systems

ABSTRACT

Vehicle arresting beds, for installation at the ends of aircraft runways, are effective to safely decelerate aircraft entering the bed. The arresting bed is assembled of a large number of blocks of cellular concrete having predetermined compressive gradient strength, so that aircraft landing gear is subjected to drag forces effective to slow a variety of types of aircraft, while providing deceleration within a safe range of values. An arresting bed typically includes an entry region of a depth increasing from 9 to 24 inches formed of blocks having a first compressive gradient strength. A second region, which may be tapered into the first region and increase in depth to 30 inches, is formed of blocks having a greater compressive gradient strength. An aircraft thus experiences increasing drag forces while it travels through the bed, to provide an arresting capability suitable for a variety of aircraft. A protective hardcoat layer of cellular concrete of strength greater than the blocks overlays the blocks to enable service personnel to walk on the bed without damage. Arresting bed systems may be provided in alternative configurations, such as a bed formed of an aggregate including pieces of cellular concrete with or without interspersed pieces of other compressible material and covered by a hardcoat layer.

RELATED APPLICATIONS

(Not Applicable)

FEDERALLY SPONSORED RESEARCH

(Not Applicable)

BACKGROUND OF THE INVENTION

This invention relates to systems for slowing travel of vehicles and,more particularly, to cellular concrete arresting bed systems to safelydecelerate an aircraft which runs off the end of a runway.

Aircraft can and do overrun the ends of runways raising the possibilityof injury to passengers and destruction of or severe damage to theaircraft. Such overruns have occurred during aborted take-offs or whilelanding, with the aircraft traveling at speeds to 80 knots. In order tominimize the hazards of overruns, the Federal Aviation Administration(FAA) generally requires a safety area of 1,000 feet in length beyondthe end of the runway. Although this safety area is now an FAA standard,many runways across the country were constructed prior to its adoptionand are situated such that water, roadways or other obstacles preventeconomical compliance with the one thousand foot overrun requirement.

Several materials, including existing soil surfaces beyond the runwayhave been assessed for their ability to decelerate aircraft. Soilsurfaces are very unpredictable in their arresting capability becausetheir properties are unpredictable. For example, very dry clay can behard and nearly impenetrable, but wet clay can cause aircraft to miredown quickly, cause the landing gear to collapse, and provide apotential for passenger and crew injury as well as greater aircraftdamage.

A 1988 report addresses an investigation by the Port Authority of NewYork and New Jersey on the feasibility of developing a plastic foamarrestor for a runway at JFK International Airport. In the report, it isstated that analyses indicated that such an arrestor design is feasibleand could safely stop a 100,000 pound aircraft overrunning the runway atan exit velocity up to 80 knots and an 820,000 pound aircraftoverrunning at an exit velocity up to 60 knots. The report states thatperformance of an appropriate plastic foam arrestor configuration wasshown to be potentially "superior to a paved 1,000 foot overrun area,particularly when braking is not effective and reverse thrust is notavailable." As is well known, effectiveness of braking may be limitedunder wet or icy surface conditions. (University of Dayton reportUDR-TR-88-07, January 1988.)

More recently, an aircraft arresting system has been described in U.S.Pat. No. 5,193,764 to Larrett et al. In accordance with the disclosureof that patent, an aircraft arresting area is formed by adhering aplurality of stacked thin layers of rigid, friable, fire resistantphenolic foam to each other, with the lower-most layer of foam beingadhered to a support surface. The stacked layers are designed so thatthe compressive resistance of the combined layers of rigid plastic foamis less than the force exerted by the landing gear of any aircraft ofthe type intended to be arrested when moving into the arresting areafrom a runway so that the foam is crushed when contacted by theaircraft. The preferred material is phenolic foam used with a compatibleadhesive, such as a latex adhesive.

Tests of phenolic foam based arrestor systems indicate that while suchsystems can function to bring aircraft to a stop, the use of the foammaterial has disadvantages. Major among the disadvantages is the factthat foam, depending upon its properties, can typically exhibit arebound property. Thus, it was noted in phenolic foam arresting bedtesting that some forward thrust was delivered to the wheels of theaircraft as it moved through the foamed material as a result of therebound of the foam material itself.

Foamed or cellular concrete as a material for use in arresting bedsystems has been suggested and undergone limited field testing in theprior art. Such testing has indicated that cellular concrete has goodpotential for use in arresting bed systems, based on providing many ofthe same advantages as phenolic foam while avoiding some of phenolicfoam's disadvantages. However, the requirements for an accuratelycontrolled crushing strength and material uniformity throughout thearresting bed are critical and, so far as is known, the production ofcellular concrete of appropriate characteristics and uniformity has notpreviously been achieved or described. Production of structural concretefor building purposes is an old art involving relatively simple processsteps. Production of cellular concrete, while generally involving simpleingredients, is complicated by the nature and effect of aeration, mixingand hydration aspects, which must be closely specified and accuratelycontrolled if a uniform end product, which is neither too weak nor toostrong, is to be provided for present purposes. Discontinuities,including areas of weaker and stronger cellular concrete, may actuallycause damage to the vehicle that is being decelerated if, for example,deceleration forces exceed wheel support structure strength. Suchnonuniformity also results in an inability to accurately predictdeceleration performance and total stopping distance. In one recentfeasibility test utilizing commercial grade cellular concrete, anaircraft instrumented for recording of test data taxied through a bedsection and load data was acquired. Even though steps had been taken totry to provide production uniformity, samples taken and aircraft loaddata from the test arresting bed showed significant variations betweenareas where the crush strength was excessively high and areas where itwas excessively low. Obviously, the potential benefit of an arrestorsystem is compromised, if the aircraft is exposed to forces that coulddamage or collapse the main landing gear.

A 1995 report prepared for the Federal Aviation Administration entitled"Preliminary Soft Ground Arrestor Design for JFK International Airport"describes a proposed aircraft arrestor. This report discusses thepotential for use of either phenolic foam or cellular concrete. As tophenolic foam, reference is made to the disadvantage of a "rebound"characteristic resulting in return of some energy following compression.As to cellular concrete, termed "foamcrete", it is noted that "aconstant density (strength parameter) of foamcrete is difficult tomaintain" in production. It is indicated that foamcrete appears to be agood candidate for arrestor construction, if it can be produced in largequantities with constant density and compressive strengths. Flat platetesting is illustrated and uniform compressive strength values of 60 and80 psi over a five to eighty percent deformation range are described asobjectives based on the level of information then available in the art.The report thus indicates the unavailability of both existing materialshaving acceptable characteristics and methods for production of suchmaterial, and suggests on a somewhat hypothetical basis possiblecharacteristics and testing of such materials should they becomeavailable.

Thus, while arresting bed systems have been considered and some actualtesting of various materials therefor has been explored, practicalproduction and implementation of an arresting bed system which, withinspecified distances, will safely decelerate aircraft of known size andweight moving at a projected rate of speed off of a runway, has not beenachieved. The particular material to be used, as well as theconfiguration and fabrication of an arresting bed, are all critical tothe provision of an effective arresting bed system. To provide aneffective arresting bed for vehicles of a range of sizes, weights andbed entry speeds, requires use of bed designs, materials and fabricationtechniques capable of providing predictable drag effects and rates ofvehicle deceleration. Computer program models or other techniques may beemployed to develop drag or deceleration objectives for arresting beds,based on calculated forces and energy absorption for aircraft ofparticular size and weight, in view of corresponding landing gearstrength specifications for such aircraft. However, such objectivesremain merely an abstract goal in the absence of effective bedconfigurations, materials and fabrication techniques appropriate toconvert arresting bed objectives into reality to achieve the desiredresults. As a result, prior information as to potential arresting bedmaterials and deceleration objectives has been inadequate to enablefabrication of a practical arresting bed suitable for use by commercialpassenger aircraft and other vehicles.

Objects of the invention are, therefore, to provide new and improvedvehicle arresting bed systems and such systems having one or more of thefollowing advantages and capabilities:

assembly from pre-cast cellular concrete which has been acceptancetested;

block or aggregate assembly enabling progressive variation of both depthand compressive strength characteristics;

predetermined arresting characteristics, substantially independent ofweather conditions;

long-life weather resistant construction;

hardcoat covering to support pedestrian access;

ability of crash/fire/rescue vehicles to fully maneuver on an arrestingbed;

ease of exit by passengers from a vehicle which has entered an arrestingbed; and

ease of repair by block or aggregate replacement following use by anoverrunning vehicle.

SUMMARY OF THE INVENTION

In accordance with the invention, a vehicle arresting bed systemincludes an initial section including first and second lateral rows ofblocks of cellular concrete having a first dry density in the lowerportion of a range of 12 to 22 pcf. Blocks of the second row have aheight incrementally greater than the height of blocks of the first row.Also, the blocks of the first and second rows have a first compressivegradient strength to provide vehicle deceleration.

The bed system has a further section including third and fourth lateralrows of blocks of cellular concrete having a second dry density greaterthan the first dry density. Blocks of the fourth row have a heightincrementally greater than the height of blocks of the third row and theblocks of the third and fourth rows have a second compressive gradientstrength, greater than the first compressive gradient strength, toprovide greater vehicle deceleration.

A hardcoat layer overlays blocks of the first, second, third and fourthrows. The hardcoat layer comprises cellular concrete having a drydensity greater than the first and second dry densities and a thicknessnot exceeding ten percent of the average height of the blocks.

A vehicle arresting bed system may desirably have additionalcharacteristics, such as the following. The blocks are preferably formedof cellular concrete having a wet density in a range of 15 to 23 pcf andcured in forms of predetermined sizes. By way of example, all of theblocks in a preferred embodiment are of the same length and width, butsome are of different predetermined heights, with a first section ofblocks having a 60/80 compressive gradient strength and a second sectionof blocks having an 80/100 compressive gradient strength. The hardcoatlayer may be formed of cured-in-place cellular concrete having greaterstrength to provide overall protection of the arresting bed and permitmaintenance personnel to walk on the bed without damaging it.

For a better understanding of the invention, together with other andfurther objects, reference is made to the accompanying drawings and thescope of the invention will be pointed out in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are respectively a plan view, and longitudinal andtransverse cross-sectional views, of a vehicle arresting bed systemusing block construction in accordance with the invention.

FIGS. 2A and 2B are respectively portions of similar plan andlongitudinal views of a vehicle arresting bed system using aggregatefabrication in accordance with the invention.

FIG. 3 shows dimensions of a typical block of cellular concrete suitablefor use in an arresting bed system.

FIGS. 4, 5 and 6 show alternative constructions of cellular concreteblocks.

FIGS. 7 and 8 show test results in terms of compressive force versuspercentage of penetration for samples of cellular concrete of twodifferent strengths.

DETAILED DESCRIPTION OF THE INVENTION

The use of cellular concrete in arresting bed applications requires thematerial to be generally uniform in its resistance to deformation sinceit is the predictability of forces acting on the surface of contactingmembers of the vehicle which is being stopped that allows the bed to bedesigned, sized and constructed in a manner which will ensure acceptableperformance. In order to obtain such uniformity, there must be carefulselection and control of the ingredients used to prepare the cellularconcrete, the conditions under which it is processed, and its curingregime.

The ingredients of cellular concrete are generally a cement, preferablyPortland cement, a foaming agent, and water. Very fine sand or othermaterials can also find application in some circumstances, but are notused in presently preferred embodiments. The currently preferred type ofcement for arresting bed application is Type III Portland cement. Forpresent purposes, the term "cellular concrete" is used as a generic termcovering concrete with relatively small internal cells or bubbles of afluid, such as air, and which may include sand or other material, aswell as formulations not including such sand or other material.

Construction of the arresting bed system can be accomplished byproducing the cellular concrete at a central production facility or atthe site of the bed and pouring the concrete into forms of appropriatedimensions to achieve the desired geometry for the system. However, inthe interests of uniformity of material characteristics and overallquality control, it has currently been found preferable to cast sectionsof the overall bed using forms of appropriate size and then transportthe sections to the site and install them to provide the overallconfiguration of the bed. In the latter case, such units or sections, inthe form of blocks of predetermined sizes, can be produced and helduntil completion of quality control testing. The blocks can then beplaced at the site and adhered to the runway safety area using asphalt,cement grout, or other suitable adhesive material, depending on theconstruction materials of the safety area itself.

In either case, in accordance with the invention a hardcoat is appliedto the outer surface of the assembled arresting bed to provide astronger surface that is not as easily deformed as the major structureof the bed itself. This allows maintenance to be performed withoutserious deformation damage to the main structure. A preferred hardcoatconsists of foamed concrete wherein the wet densities are somewhathigher, for example in the range of about 22 to about 26 lbs./cu. ft.Finally a weather resistant protective film or paint can be applied togive the structure a desired visual appearance and act as protectionagainst weather degradation. Preferred coatings include water basedsilicone materials.

DEFINITION OF "COMPRESSIVE GRADIENT STRENGTH" OR "CGS"

The term "compressive strength" (not CGS) is normally understood to meanthe amount of force (conventionally measured in pounds per square inch)which, when applied at a vector normal to the surface of a standardizedsample, will cause the sample to fail. Most conventional test methodsspecify test apparatus, sampling procedures, test specimen requirements(including size, molding, and curing requirements) rates of loading andrecord keeping requirements. An example is ASTM C 495-86 "StandardMethod for Compressive Strength of Lightweight Insulating Concrete."While such conventional test methods are useful when designingstructures that are required to maintain structural integrity underpredicted load conditions (i.e., have at least a minimum strength), theobject of arresting bed systems is to fail in predictable specifiedmanner, thereby providing controlled, predictable resistive force as thevehicle deforms the cellular concrete (i.e., a specific compressivegradient strength). Thus, such conventional testing focuses ondetermining strength up to point of failure, not strength duringcompressive failure. Stated more simply, knowing what amount of forcewill shatter a specimen of cellular concrete material does not answerthe critical question of what amount of drag or deceleration will beexperienced by a vehicle moving through an arresting bed system. Incontrast to a "one time" fracture strength as in the prior art, forpresent purposes testing must evaluate a continuous compressive failuremode as a portion of a specimen is continuously compressed to abouttwenty percent of its original thickness. Equipment and methods suitablefor such continuous testing as appropriate for present purposes havegenerally not been previously available.

Because of the wide range of variables available in materials andprocessing of cellular concretes, and the size and cost of constructingarresting beds for testing, it is imperative that accurate testinformation be available to predict the amount of resistive force aparticular variety of cellular concrete, processed and cured in acertain way, will provide when used in an arresting bed system. Bydeveloping new test methodology to focus the resulting data onmeasurement of the resistive force occurring during continuouscompressive failure of a sample, instead of simple one-time "compressivestrength", new test methods and apparatus have been developed to enablereliable testing and confirmation of appropriate cellular concretematerials and process variables.

As a result it has been determined that the compressive force needed tocrush cellular concrete to 20 percent of its original thickness varieswith the depth of penetration. This characteristic, which the presentinventors term "compressive gradient strength" or "CGS" must beaccurately specified in order to construct a cellular concrete vehiclearresting bed having known deceleration characteristics to safelydecelerate an aircraft. Thus, a penetration type test method where thecompressive strength of a sample of cellular concrete is gauged not byapplying a force that will fracture a sample, but rather willcontinuously provide data on resistive forces generated as a test probehead having a specified compressive contact surface is moved through avolume of cellular concrete, is key to obtaining the data necessary toformulate and use cellular concrete in arresting bed applications. Asthus measured, CGS will vary over a range with penetration depth,resulting in a gradient value (such as 60/80 CGS with an average CGS of70 psi over the penetration range) rather than a simple singularfracture value as in prior tests.

For present purposes, the term "compressive gradient strength" (or"CGS") is used to refer to the compressive strength of a section ofcellular concrete from a surface and continuing to an internal depth ofpenetration, which may typically be 66 percent of the thickness of thesection. As defined, CGS does not correspond to compressive strength asdetermined by standard ASTM test methods. Test methods and apparatussuitable for determining CGS are disclosed in application Ser. No.08/796,968, filed concurrently herewith, having a common assignee, andhereby incorporated herein by reference.

ARRESTING BED OF FIGS. 1A, 1B and 1C

With reference to FIG. 1 (collectively including FIGS. 1A, 1B and 1C),there is illustrated an embodiment of a vehicle arresting bed system inaccordance with the invention. As shown in FIG. 1A, the bed has a lengthand width and also a thickness as shown in FIGS. 1B and 1C. The bed isconfigured to decelerate an aircraft entering the bed from the left inFIG. 1A. Basically, the FIG. 1 system is constructed of pre-cast blocksof cellular concrete having two different compressive gradient strengthsand a variety of different thicknesses, with intended installation atthe end of an airport runway. Subsurface 50 supporting the system shouldtypically be relatively flat, smooth and level (subject to having aslope appropriate for water runoff requirements) and capable ofsupporting aircraft which leave the runway. Subsurface 50 should be ingood condition and cleaned satisfactorily for placement and bonding ofthe arresting bed system. To show vertical details, the verticaldimensions of FIGS. 1B and 1C are expanded relative to the dimensions ofFIG. 1A (e.g., the width of the bed in FIG. 1A may typically be 150feet, while the maximum thickness of the bed in FIGS. 1B and 1C maytypically be 30 inches). Also, certain dimensions, such as block size,are distorted for clarity of illustration (e.g., rather than show thethousands of blocks actually included in a typical arresting bed).

A typical block suitable for use in the FIG. 1 system is illustrated inFIG. 3. As shown, block 70 may be fabricated by placing wet cellularconcrete in curing forms of uniform width 74 (typically 4 feet) andlength 76 (typically 8 feet). Block thickness 72 may be varied in arange of 8 to 30 inches, for example, to provide blocks having heightsvarying in increments (typically of from 3/4 inch increments of heightfor a fine taper to increments of 3 inches) in order to enable provisionof front to rear tapered bed configurations able to providepredetermined incremental increases in drag forces. In the blockembodiment shown in FIG. 3, there are included transverse lifting slots78 and 80. Slots 78 and 80, suitable for use with a fork lift type oflifting mechanism, are formed by placing lightweight rectangular plasticsleeves in the bottom of a form when casting the block. Other blockfeatures and embodiments usable in arresting beds constructed inaccordance with the invention will be discussed with reference to FIGS.4, 5 and 6.

As shown, the FIG. 1 vehicle arresting bed system has a bed area ofcellular concrete which includes a first section 52, comprising anassembly of blocks having a first CGS and a first dry density, and asecond section 54, comprising an assembly of blocks having a second CGSand a second dry density. As shown in the side sectional view of FIG.1B, sections 52 and 54 partially overlap (in what might be consideredsection 52/54), with a darkened line indicating the juncture whereincertain blocks of section 52 overlie blocks of section 54 in atransition region. In a particular embodiment, the section 52/54 blocksmay actually be composite blocks (i.e., single blocks including a 52portion having a first CGS and also a 54 portion having a second CGS).In other embodiments separate blocks of different CGS may be stacked forsection 52/54.

More particularly, vehicle arresting bed systems of the type illustratedin FIG. 1 include a first lateral row of blocks (e.g., row 52a) ofcellular concrete having a first CGS and a first dry density in a rangeof 13 to 18.5 pounds per cubic foot (pcf). Each of the blocks in firstrow 52a has a first height and is fabricated to be verticallycompressible to a compressed height (e.g., typically compressible toabout 20 percent of initial thickness). These blocks may be fabricatedto exhibit a 60/80 CGS characteristic as represented in FIG. 7, whichwill be discussed below. As shown in FIGS. 1A and 1B, the first section52 includes a second row 52b and a plurality of additional lateral rowsillustrated as rows 52c through 52n, formed of cellular concrete havingthe same basic characteristics as in the blocks of row 52a, but some ofwhich differ row-to-row by an incremental height differential. Also, asdiscussed with reference to overlap section 52/54 certain rows ofblocks, such as row 52n, overlay blocks of row 54d on a composite blockor stacked block basis. In this embodiment successive 3/4 inch changesin thickness were utilized in section 52 to provide tapered or slopingcharacteristics resulting in gradually increasing vehicle arrestingcapabilities. Corresponding 3 inch changes in thickness were utilized insection 54, in this particular design.

Arresting bed systems of the type illustrated also include a thirdlateral row 54g of blocks of cellular concrete having a second drydensity which may be at a higher level in the same range as the firstblocks in section 52. As shown, lateral row 54g is positioned parallelto and to the rear of the first lateral row 52a. Row 54g is in turnfollowed by a lateral row 54h of incrementally greater height. Theblocks of section 54 are fabricated to be vertically compressiblesubject to a second compressive gradient strength, which will generallybe specified to exceed the CGS of the blocks of section 52. These blocksmay be fabricated to exhibit a 80/100 CGS characteristic, as representedin FIG. 8 which will be discussed below, and a dry density in a range of16 to 21.5 pcf. In the illustrated embodiment the first row of blocks54a of section 54 includes only a single course or layer of the secondCGS. Successive rows of section 54 include increasing thickness of thesecond CGS material, until the section 54 blocks reach the full heightof the arresting bed beyond section 52. Successive rows of section 54then increase in thickness by 3 inch increments in advance of reachingmaximum height in a rear level portion comprising rows of the samethickness continuing to final rear row 54n. Rows of increased height,such as row 54n, may be formed of two or three superimposed blocks ofreduced thickness or of rows of single relatively thick blocks,depending upon fabrication, handling and site delivery considerations.

FIG. 7 illustrates the CGS characteristics of a cellular concrete samplerepresentative of a block from section 52 of FIG. 1, as determined bytest. In FIG. 7, the bottom scale represents percentage of test probepenetration expressed in tenths of sample thickness or height. Thevertical scale represents test probe compressive force expressed inpounds per square inch (psi). The test data of principal interest istypically within the range of penetration from 10 to 60 percent ofsample thickness. Data outside this range may be less reliable, withcrushed material build-up effects occurring beyond about 70 percentpenetration.

As illustrated in FIG. 7, the failure strength of cellular concreteexhibits a gradient with resistance to compression increasing with depthof penetration. For a particular design of an arresting bed asillustrated in FIG. 1, the line through points A and B in FIG. 7represents a generalized 60/80 CGS, i.e., a CGS characterized by acompression strength changing from approximately 60 psi to approximately80 psi over a 10 to 66 percent penetration range. The average, over thisrange is thus nominally equal to 70 psi at mid-point C. In FIG. 7, theline joining points A and B represents a typical generalized compressivestrength gradient line for blocks of section 52 of FIG. 1. Lines D and Erepresent quality control limits and line F represents actual test dataas recorded for a specific test sample of cellular concrete. In thisexample, a test sample for which test data over a 10 to 60 percentpenetration range remains within quality control limit lines D and E,represents an arresting block fabricated within acceptable tolerances.FIG. 8 is a similar illustration of CGS characteristics of a test sampleof a block suitable for use in section 54 of FIG. 1, having an 80/100CGS which is nominally equal to 90 psi, when averaged over a selecteddepth of penetration (e.g., a 10 to 66 percent penetration range). Forpresent purposes, "nominal" or "nominally" is defined as referring to avalue or relationship which is within about plus or minus 15 percent ofa stated value or relationship.

As shown, the FIG. 1 system further includes an inclined entrance ramp56 positioned across the vehicle entrance front side of the firstlateral row 52a. The ramp, which may be formed of asphalt mix or otherpermanent type material, tapers up to a height adjacent the blocks ofrow 52a, which is typically greater than the compressed height of theblocks of row 52a. In a particular embodiment, a ramp height of about 3inches was used adjacent to 8 inch blocks having an estimated minimumcompressed height of 1.8 inches. Ramp 56 is thus effective to graduallyraise an aircraft above general runway level, so that the aircraft canenter the arresting bed on a relatively smooth basis as the wheels leaveramp 56 and begin compressing the blocks of row 52a.

Also included in the FIG. 1 system is a hardcoat layer 62, in the formof a relatively thin protective layer of cellular concrete material,overlaying the blocks of both section 52 and section 54 (represented bythe uppermost boundary line of the bed in FIG. 1B). In FIG. 1A thehardcoat layer is represented as being transparent in order to showunderlying details, even though the hardcoat layer will typically not betransparent. In a preferred embodiment, hardcoat layer 62 comprises arelatively thin layer of cellular concrete having a strength to supporta pedestrian (e.g., sufficient to support a maintenance person walkingon the arresting bed) and may be covered by a weather resistant paint orsimilar coating. Layer 62 is applied over the arresting bed after allblocks of sections 52 and 54 are positioned and appropriately adhered tosupporting surface 50. Hardcoat layer 26 may typically be formed of 22to 26 pcf dry density cellular concrete with an average thickness ofabout one inch. In an arresting bed which may include blocks ranging inthickness from 8 to 20 inches or more, the thickness of hardcoat layer62 will typically not exceed 10 percent of average thickness or heightof the blocks and may be closer to 5 percent. Since the thin hardcoatlayer has relatively little effect on aircraft deceleration, testsamples typically need not be subjected to testing as described above.

As illustrated, the arresting bed system also has associated with it adebris shield 58 and service vehicle entrance ramps 60. Shield 58 may beformed of relatively light weight aluminum sheet stock adequate todeflect particles blown by jet exhaust, etc., but fragile enough toreadily yield to the tires of an aircraft. Ramps 60 are proportioned andconstructed to enable airport fire or rescue vehicles to drive up ontothe arresting bed in order to provide assistance to passengers of anaircraft which has come to a stop within the boundaries of the arrestingbed. Ramps 60 may be constructed in a stepped form, of elongated blocksof cellular concrete of appropriate strength or other suitable material.As shown, ramps 60 are constructed of blocks of square cross-sectionaldimensions able to be accommodated by a fire or rescue vehicle drivingonto the bed.

In a typical arresting bed installation, appropriate for arrestingtravel of a variety of types of aircraft, the blocks of section 52 maytypically have thicknesses varying in 3/4 inch increments from 9 inchesto 24 inches, a dry density of about 17 pcf, and provide a 60/80 CGS asdescribed above. The blocks of section 54 may correspondingly havethicknesses varying in three inch increments from 24 inches to 30inches, a dry density of about 19 pcf, and provide an 80/100 CGS. Infabrication of the blocks, the blocks of section 52 may be formulatedfrom cellular concrete having a wet density toward the lower portion ofa range of about 14 to 23 pcf, with the blocks of section 54 fabricatedfrom cellular concrete having a wet density toward the upper portion ofsuch range. The composite blocks in section 52/54 would correspondinglyconsist partially of 60/80 CGS material and partially of 80/100 CGSmaterial. Overall, sections 52 and 54 may have an aggregate length of400 feet, a width of 150 feet and front end and rear end thicknesses of9 inches and 30 inches, respectively. It will be appreciated that forany particular implementation of the invention, performance achievedwill be dependent upon the characteristics of the materials andarresting system design as specified and fabricated in order to meetidentified site-specific performance objectives. Parameters relating tomaterials or systems for any specific implementation are beyond thescope of present purposes and specific values are discussed only asgeneral examples of possible parameter magnitudes.

As described, the two major sections 52 and 54 can be constructed bycontiguous assembly of preformed blocks which are then grouted orotherwise adhered to the support surface. Alternatively, other forms ofconstruction may be employed in accordance with the invention. Forexample, with appropriate process control, an arresting bed similar tothat illustrated can be poured and cured in place on a unitary orsectioned basis. Another form of construction is illustrated in FIG. 2(comprising FIGS. 2A and 2B).

Referring now to FIGS. 2A and 2B, there is shown a portion of a vehiclearresting bed system in accordance with the invention, which includes abed 90 formed of an aggregate including pieces of cellular concrete. Forpresent purposes, and consistent with its dictionary definitions,"aggregate" is defined as a mass or volume of material formed ofhomogeneous or non-homogeneous units, pieces or fragments of the same ordifferent sizes and of regular or irregular shapes. Pursuant to theinvention, an aggregate as used in bed 90 may consist entirely of piecesof cellular concrete, typically having dimensions smaller thanone-quarter of the average bed thickness, or may comprise pieces ofcellular concrete with other material mixed in. Such other material mayinclude pieces of phenolic foam or other compressible material, hollowglass spheres, hollow ceramic spheres, or other crushable items ofselected material and shape. As shown, bed 90 has length, width andthickness and is configured to decelerate a vehicle, such as anaircraft, entering the bed from the left. More particularly, asrepresented in FIG. 2B, the aggregate of bed 90 is arranged to increasein thickness from left to right, so that some portions have differentthickness than other portions. In addition, at 90a there is indicated asloping portion of aggregate which may have a higher compressibilitythan the partially overlying aggregate portion to the left in FIG. 2B.The bed may thus include portions having different compressibility sothat vehicle drag or deceleration increases as a vehicle travels throughthe bed.

The arresting bed system of FIGS. 2A and 2B includes edge members 92 and94 along the perimeter of bed 90 to constrain the aggregate fromspreading beyond the desired length and width of the bed. Asillustrated, the edge members are blocks of cellular concrete similar tothose described above and each having a suitable CGS. In FIG. 2A, eachedge member 92 and 94 includes a row of blocks and the complete bedsystem would have a suitable overall length, with an additional row ofblocks across the right hand end of the bed. The arresting bed system,as illustrated, also includes a stabilizing layer, represented by line96 in FIG. 2B, overlaying the bed 90 to limit movement of the aggregatewithin the bed. Stabilizing layer 96 may typically be a relatively thinhardcoat layer of cellular concrete as described above. In FIG. 2A thestabilizing layer is represented as being transparent in order to showunderlying details.

FIGS. 4, 5 and 6 illustrate particular embodiments of cellular concreteblocks usable in arresting bed systems pursuant to the invention. Theblock of FIG. 4 is a composite block including an upper portion 100 ofcellular concrete having a desired CGS and a thin lower layer 102 ofstronger cellular concrete or other material to provide added strength,particularly during block transport and installation. FIG. 5 shows ablock of cellular concrete 104 which includes within its lower portionreinforcing members, illustrated as grid of suitable fiber, metal orother material. FIG. 6 illustrates a block 108 of cellular concretecontaining within it crushable pieces or forms of other material. Asrepresented in somewhat idealized form, such material may comprise oneor more of: regular or irregular pieces of compressible material; glassor ceramic spheres; hollow items of selected material and shape; orother suitable pieces. Such items or materials will typically bepositioned near the bottom or distributed through out the block and haveminor effect in decelerating a vehicle, or be taken into account indetermining CGS, or both.

The nature of a cellular concrete arresting bed system is such that itsconstruction will inherently be relatively time consuming and expensive.Therefore, it is important that the method and information used todesign the system be reliable enough to correlate with and predictperformance under actual conditions of use. The use of a computerizedmodeling program, data obtained from appropriate test methodology, orboth, can provide the necessary correlation between prediction and fieldresults.

In general, to be effective a computer modeling program must be arrangedto accept data as to aircraft weight, center of gravity, moment ofinertia, landing gear structure and stress capacity and projected speedsat entry into the bed. The specifics of a selected bed geometry andmaterial strength relative to the crushing of the arresting bed as thevehicle moves through are typically also inputs into the program. Theprogram would then be configured to use that information to provideoutput data regarding deceleration versus distance and resulting loadson nose and main landing gear at different speeds.

The necessary material strength information for the program can beprovided in one of two ways. First, actual test information using testmethodology for samples of cellular concrete, can be used in theprogram. In this manner, the program accepts the materialcharacteristics of a selected formulation of cellular concrete as fixedinformation and determines results based on that information.Alternatively, it can be assumed that the cellular concrete to be usedwill exhibit a certain characteristic drag force. Then, the designers ofthe arresting bed can use the above described testing methodology toidentify cellular concrete formulas, processing techniques, and curingregimes that will result in materials that match the requirements forthe design.

As an alternative to a comprehensive computer modeling program,arresting bed design can be more closely based on pro forma testing. Bedsections can be constructed for testing using cellular concrete of oneor more compressive failure strengths. Aircraft, instrumented wheelstructures or other compressive structures can then be driven intosample bed sections and resulting bed performance can then be determinedand utilized in design of a complete arresting bed. Many otheralternatives and variations will become apparent to skilled personshaving an understanding of the invention. For example, beds or sectionsthereof may be of uniform or varying thickness, may have gradual orstepped thickness variation, may be of uniform or multiple CGS, may beof unitary or stacked blocks or aggregate, and may be of selected widthand overall length, as suited for particular applications and use byparticular aircraft or other vehicles.

While there have been described the currently preferred embodiments ofthe invention, those skilled in the art will recognize that other andfurther modifications may be made without departing from the inventionand it is intended to claim all modifications and variations as fallwithin the scope of the invention.

What is claimed is:
 1. A vehicle arresting bed system, comprising:a bedof cellular concrete having length, width and thickness and configuredto decelerate a vehicle entering said bed; and a hardcoat layeroverlaying said bed, said hardcoat layer comprising cellular concretehaving a thickness not exceeding ten percent of average thickness ofsaid bed and a strength to support a pedestrian.
 2. A vehicle arrestingbed system as in claim 1, wherein said bed is formed of cellularconcrete having a compressibility which is different at different pointsalong the length of said bed.
 3. A vehicle arresting bed system as inclaim 1, wherein said bed comprises lateral rows of blocks of cellularconcrete.
 4. A vehicle arresting bed system as in claim 3, wherein saidbed is assembled from preformed blocks of a common length and a commonwidth and includes blocks of different thickness.
 5. A vehicle arrestingbed system as in claim 1, wherein said bed comprises an aggregateincluding pieces of cellular concrete and said bed system additionallyincludes edge members arranged to constrain said aggregate fromspreading beyond said length and width.
 6. A vehicle arresting bedsystem as in claim 5, wherein said aggregate consists of pieces havingdimensions not exceeding ten percent of average thickness of said bedand said cellular concrete pieces are characterized by one of: irregularsize and shape; common size and shape.
 7. A vehicle arresting bed systemas in claim 5, wherein said edge members are formed of blocks ofcellular concrete positioned along the perimeter of said bed.
 8. Avehicle arresting bed system as in claim 5, wherein said bed includescellular concrete pieces having a compressibility which is different atdifferent points along the length of said bed.
 9. A vehicle arrestingbed system, comprising:a bed of cellular concrete having length, widthand thickness and including lateral rows of blocks of cellular concrete,each block having a predetermined compressive gradient strength over adepth of penetration to provide gradual deceleration of a vehicle; and ahardcoat layer overlaying said bed, said hardcoat layer comprisingcellular concrete having a thickness not exceeding ten percent ofaverage thickness of said bed and a strength to support a pedestrian.10. A vehicle arresting bed system as in claim 9, wherein said bedincludes a first lateral row of blocks having a dry density nominallyequal to 17 pcf and a second lateral row of blocks having a dry densitynominally equal to 19 pcf.
 11. A vehicle arresting bed system as inclaim 10, wherein blocks of said second lateral row have a greaterthickness than blocks of said first lateral row.
 12. A vehicle arrestingbed system as in claim 10, wherein blocks of said first lateral row havea 60/80 compressive gradient strength nominally equal to 70 psi andblocks of said second lateral row have an 80/100 compressive gradientstrength nominally equal to 90 psi, when averaged over a depth ofpenetration within said respective blocks.
 13. A vehicle arresting bedsystem as in claim 9, wherein said bed includes blocks having a 60/80compressive gradient strength nominally equal to 70 psi, when averagedover a depth of penetration of said blocks.
 14. A vehicle arresting bedsystem as in claim 9, wherein said bed includes blocks having a drydensity in a range of 12 to 22 pcf.
 15. A vehicle arresting bed systemas in claim 9, wherein said bed includes blocks formed of cellularconcrete having a wet density in a range of 14 to 23 pcf cured in formsof predetermined sizes.
 16. A vehicle arresting bed system as in claim9, wherein said bed includes blocks of cellular concrete having embeddedtherein compressible pieces of a material other than cellular concrete.17. A vehicle arresting bed system as in claim 9, wherein said bedincludes blocks of cellular concrete having embedded therein one of:hollow glass spheres; hollow ceramic spheres; hollow items of selectedmaterial and shape.
 18. A vehicle arresting bed as in claim 9, whereinsaid bed includes blocks of cellular concrete which include a lowerlayer of higher strength material.
 19. A vehicle arresting bed as inclaim 9, wherein said bed includes blocks of cellular concrete havingreinforcing members embedded therein.
 20. A vehicle arresting bed as inclaim 19, wherein said reinforcing members comprise a grid.
 21. Avehicle arresting bed system comprising:a bed formed of an aggregateincluding pieces of cellular concrete, said bed having length, width andthickness and configured to decelerate a vehicle entering said bed; edgemembers positioned along the perimeter of said bed to constrain saidaggregate from spreading beyond said length and width; and a stabilizinglayer overlaying said bed to limit movement of said aggregate withinsaid bed, said stabilizing layer being a hardcoat layer of cellularconcrete having a thickness not exceeding ten percent of averagethickness of said bed and a strength to support a pedestrian.
 22. Avehicle arresting bed system as in claim 21, wherein said edge membersare formed of cellular concrete blocks, positioned along said perimeter.23. A vehicle arresting bed system as in claim 21, wherein said cellularconcrete pieces are characterized by one of: irregular size and shape;common size and shape; varied size and shape.
 24. A vehicle arrestingbed system as in claim 21, wherein said aggregate additionally includescrushable pieces of a material other than cellular concrete.
 25. Avehicle arresting bed system as in claim 21, wherein said aggregateadditionally includes one of: hollow glass spheres; hollow ceramicspheres; hollow items of selected material and shape.
 26. A vehiclearresting bed system as in claim 21, wherein said aggregate has adifferent thickness in different portions of said bed.
 27. A vehiclearresting bed system as in claim 21, wherein said aggregate has adifferent compressibility in different portions of said bed.
 28. Avehicle arresting bed system, comprising:a first lateral row of blocksof cellular concrete having a first dry density in a range of 12 to 22pcf, blocks of said first row having a first height; a second lateralrow of blocks of cellular concrete having a dry density nominally thesame as said first dry density, blocks of said second row having aheight incrementally greater than said first height; a hardcoat layeroverlaying blocks of said first and second rows, said hardcoat layercomprising cellular concrete having a dry density greater than saidfirst dry density, a strength to support a pedestrian and a thicknessnot exceeding ten percent of the average height of said blocks.
 29. Avehicle arresting bed system as in claim 21, wherein blocks of saidfirst and second rows have a predetermined compressive gradient strengthover a depth of penetration.
 30. A vehicle arresting bed system as inclaim 28, wherein blocks of said first and second rows have a 60/80compressive gradient strength nominally equal to 70 psi, when averagedover a depth of penetration of said blocks.
 31. A vehicle arresting bedsystem, comprising:first and second lateral rows of blocks of cellularconcrete having a first dry density in a range of 12 to 22 pcf, blocksof said second row having a height incrementally greater than the heightof blocks of said first row and the blocks of said first and second rowshaving a first compressive gradient strength to provide vehicledeceleration; third and fourth lateral rows of blocks of cellularconcrete having a second dry density greater than said first drydensity, blocks of said fourth row having a height incrementally greaterthan the height of blocks of said third row and the blocks of said thirdand fourth rows having a second compressive gradient strength, greaterthan said first compressive gradient strength, to provide greatervehicle deceleration; and a hardcoat layer overlaying blocks of saidfirst, second, third and fourth rows, said hardcoat layer comprisingcellular concrete having a dry density greater than said first andsecond dry densities, a strength to support a pedestrian and a thicknessnot exceeding ten percent of the average height of said blocks.
 32. Avehicle arresting bed system as in claim 31, wherein blocks of saidfirst and second rows have a 60/80 compressive gradient strengthnominally equal to 70 psi and blocks of said third and fourth rows havean 80/100 compressive gradient strength nominally equal to 90 psi, whenaveraged over a depth of penetration within said respective blocks. 33.A vehicle arresting bed system as in claim 32, additionally including atleast one lateral row of composite blocks formed partially of cellularconcrete having said 60/80 compressive gradient strength and partiallyof cellular concrete having said 80/100 compressive gradient strength.34. A vehicle arresting bed system as in claim 31, wherein said blockshave a dry density within a range of 12 to 22 pcf.
 35. A vehiclearresting bed system as in claim 31, wherein said blocks are formed ofcellular concrete having a wet density in a range of 14 to 23 pcf curedin forms of predetermined sizes.
 36. A vehicle arresting bed system asin claim 31, wherein said hardcoat layer is formed of cellular concretehaving a wet density in a range of 22 to 26 pcf, which is cured in placeoverlying said blocks.